What is bumpsteer?
As we learned the hard way, bumpsteer is a condition you want to avoid. In fact, for a car to drive and feel right, it needs to be eliminated totally. Generally, bumpsteer is caused by an incompatibility between the steering system's tie rods and the front suspension's upper and lower control arms. As the suspension cycles, the path or arc followed by the outer tie rods in relation to the control arms and spindles are different. Since something has to give, there is a toe change (the wheels toe in or out) as the suspension moves up and down. This occurs every time the suspension moves.
While bumpsteer is a factory design flaw every Mustang has to varying degrees, it becomes noticeable when the car is lowered or modified with other suspension improvements that make the car more responsive to steering input. Better tires also make bumpsteer more noticeable. When experienced, the car twitches or darts around on bumpy or undulating roads and can also be twitchy on heavy braking. The car might also pull to one side or another when braking.
Bumpsteer is eliminated from a Mustang by ensuring the tie-rod (steering linkage) pivot point matches the travel of the spindle. Match these curves by either raising the inner tie rod or lowering the outer tie-rod mounting point. Since the location of the inner tie rods and steering center link is under the oil pan, this option isn't practical. Fabrication of a spacer block to relocate and lower the outer tie-rod from its location on the spindle is easy. The Pro-Motorsports' bumpsteer corrector kit accomplishes exactly that. It repositions the outer tie rod 1-inch lower, 1-inch forward, and 1/4-inch outboard from its stock location. The 1-inch drop is for the bumpsteer, the 1-inch forward move quickens the steering the same way the longer idler and Pitman arms did on '65 Shelbys, and the 1/4-inch outboard move reduces (but doesn't eliminate) the Ackermann Angle, a toe-out condition that is designed to occur during turning. While helpful for street cars driven at low speeds, it's less desirable in racing or high-performance situations where high lateral cornering loads are present.
2 Next, loosen (but don't...
2 Next, loosen (but don't totally remove) the castle nut and separate the tie-rod end from the spindle by hitting the spindle arm with a heavy hammer. The idea is to "shock" the spindle and tie-rod end apart, which is more effective than trying to force them apart with a pickle fork or tie-rod splitter. Once the tie-rod end's stud drops through the spindle arm, remove the castle nut all the way and the tie-rod end will then easily separate from the spindle.
3 To prepare the spacer for...
3 To prepare the spacer for installation, install the large socket-head (Allen) cap screw and chamfered washer into the bottom (counterbore side) of the spacer block; this has already been done here. The block is then set into place on the tie-rod end as shown.
4 As shown here, be sure...
4 As shown here, be sure the slot in the spacer block is parallel with the cotter-pin hole in the tie-rod-end stud so the cotter pin can be reinstalled. Next, reinstall the castle nut to secure the block in place on the tie-rod end. Here, the nut on the large screw keeps it in place while the spacer is installed on the tie-rod end; it's removed once the spacer is installed.
5 Secure the tie-rod nut...
5 Secure the tie-rod nut into place. Marlo's Frame found that a 11/8-inch wrench worked perfectly for holding the spacer block while tightening the tie-rod nut. Make sure the tie rod is tight and cannot move in the tapered hole. The nut should be tightened to a minimum of 30 lbs-ft.
6 Before final assembly,...
6 Before final assembly, a fit check is required to confirm the clearance between the tie-rod stud coming through the spacer and the bottom of the spindle arm. As the Pro-Motorsports' instructions say, trimming the tie-rod stud might be required, and was in our case. Here, we're showing the stud being trimmed. Note the cotter pin has also been reinstalled.
7 Trim up to two threads...
7 Trim up to two threads from the tie-rod stud to gain adequate clearance from the spindle arm. Slightly more might be necessary, but whatever the case, be careful not to take too much of the stud away and thus cut into the cotter-pin hole.
8 On our car, we also had...
8 On our car, we also had to enlarge the holes in the spindle arm to accept the new larger cap screws that come with the bumpsteer kit. Drill out the hole with a bit that's slightly larger than the hole in the spindle arm. You may or may not need to do the same with other cars. We only had to enlarge the holes a small amount.
9 With the tie-rod stud adequately...
9 With the tie-rod stud adequately trimmed, the spacer can be installed onto the spindle. Double check your trim work: It's imperative that the tie-rod stud doesn't come in contact with the spindle arm.
10 Install a nut onto the...
10 Install a nut onto the large cap screw to hold the spacer block in place. The large washer between the spacer and spindle arm lowers the tie rod a slight bit more, which helped out even more in our application. Marlo's Frame then added another washer to go between the top of the spindle arm and the nut as shown.